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Window Condensation

DEAR TIM: I have massive amounts of window condensation forming on expensive name-brand windows in my new home. My sister has condensation on some pricey vinyl-replacement windows. The window people say it is our fault stating that the humidity is too high in our homes. I even have condensation on a storm door. What is cure for condensation, and how can I stop the water damage and mold that is forming? Patty S.

DEAR PATTY: Window condensation plagues many a homeowner each winter. Condensation can form on windows, skylights, walls, ceilings, the underside of roofs, inside closets, etc. The source of condensation is invisible water vapor that is a component of the air inside your home. Condensation on windows, even new ones, has become an increasingly more-common problem because we are building new homes better and sealing up existing homes so they are more energy-efficient.

I have people ask me frequently what is condensation. Condensation is a physical process that happens when water changes from the gaseous state to the liquid state. The air we breath and that surrounds our planet holds vast amounts of water - billions of tons. Some of this water is invisible while much of it is liquid. You can see liquid water in the sky each time you look at a cloud.

This glass has condensation on it that can peel the paint and cause both wood rot and mold if it is not stopped! PHOTO CREDIT: Tim Carter

The water vapor turns to liquid water when it comes into contact with a cool or cold surface. For condensation to form, the temperature of the surface must be at or below the dew point of the air that is touching up against the cool surface. This dew point is a moving target, and this is why the window companies are saying you and your sister might be the cause of the window condensation.

A given volume of air can only hold so much water vapor at a given temperature. The warmer the air, the more water vapor it can hold. Certainly you have experienced stifling air in the summer months where the air seems so thick you can cut it. If you had an instrument to measure relative humidity with you, you would have discovered the dew point of that air might have been as high as 72 F. This means that if there was an object outdoors whose surface was 72 F or lower, you would see water droplets forming on that relatively warm surface!

The air inside your home or that of your sister might have a dew point of 50 F. This means that if the surface temperature of the glass drops to 50 F or below, you are going to see condensation.

Even though you and your sister have state-of-the-art windows that might have Low-E insulated glass with Argon or some other inert gas between the panes of glass, you can still get window condensation. Here is why. If you keep the temperature and humidity constant inside your home as many do, condensation will form on windows as the outside temperature plummets in bitter-cold weather. The temperature of the surface of the inside pane of glass is not constant as the outdoor temperature drops! Your furnace is fighting to keep the temperature up, but it will always lose that battle if it is sized properly for your home.

You can stop or minimize condensation by monitoring the amount of water vapor in your home. As the outdoor temperature drops you must lower the amount of water in you air. Modern humidifiers have controls with outdoor sensors that can do this automatically. You should also use exhaust fans in your kitchen and bathrooms that exhaust air outdoors, not into an attic space. Indoor plants, laundry that is drying indoors, interior painting, cooking, crawl spaces that have no vapor barriers, etc. all can aggravate window condensation.

Older homes had window-condensation issues, but they were not always as bad. The reason was simple. Old homes like the one I grew up in were often very drafty. The influx of cold air mixing with the interior air made the relative humidity of the air lower. Cold air is dry by nature, and when it is mixed with warm air, the humidity drops as does the dew point! This means you would not see condensation form in an old drafty house but might see it in a new home that could be built next door.

Condensation on a storm door is very common. Storm doors are installed to act as a water and wind barrier. They often have single-pane glass and the temperature of that glass is frequently equal to the actual outdoor temperature. Moist warmer air from inside your home leaks past your primary door and contacts the cold storm-door glass. Condensation starts to form in seconds if the storm door is sealed well as it should be.

Fans that blow air on window condensation can help evaporate the liquid water putting it back into the gaseous state. But always remember that the light fog you see in the first phase of window condensation is doing the same thing a blaring smoke detector does; it is screaming at you warning that you have too much water in your air.